🐛 Use ::sm/any instead of any for get-file-fragment rpc method schema

The usage of `any?` predicate as-is uses the standard any generator that
causes to generate java.lang.Character instances created that are not
properly serialiable to JSON. The `::sm/any` schema delimits the
generator to a commonly known serializable types on json.

backend/src/app/rpc/commands/files.clj

@@ -357,7 +357,7 @@
    [:id ::sm/uuid]
    [:file-id ::sm/uuid]
    [:created-at ::dt/instant]
-   [:content any?]])
+   [:content ::sm/any]])
 
 (def schema:get-file-fragment
   [:map {:title "get-file-fragment"}